Annealing furnace



Feb. 3, 1959 E. GUENZI ANNEALING FURNACE 2 Sheets-Sheet 1 Filed June 2, 1955 Own/6m r; Era/4b ue/vz :+form z Feb. 3, 1959 EVGUENZI 2,372,175

ANNEALING FURNACE Filed June 2, 1955 2 Sheets-Sheet 2 jam AH my ANNEALING FURNACE Eraldo Guenzi, Pont-a-Mousson, France, assignor to 'Compagnie de Pont-a-Mousson, Nancy, France, a French body corporate Application June 2, 1955, Serial No. 512,798

Claims priority, application France February 14, 1955 1 *Claim. (Cl. 263-8) The present invention relates to furnaces for annealing pipes, tubular objects or other objects cast in a metal mould by centrifugal casting or any other process, and more particularly to such objects composed of spheroidal graphite cast iron.

Generally, pipes and various other objects of cast iron obtained by centrifugal casting in a metal mould must be subjected to a heat treatment for the purpose of decomposing the carbides, the presence of which is due to the rapid cooling of the cast iron.

In the case of spheroidal graphite cast irons, the heat treatment generally comprises two successive structural transformations a. A first transformation, termed graphitization, intended to decompose the cementite contained in the cast iron; this is the case when the solidification is very rapid owing to casting in a metal mould.

b. A second transformation, termed ferritization, intended to obtain a more or less complete decomposition of the pearlite into ferrite with liberation of carbon in the form of graphite; the cast iron varies considerably as concerns its mechanical characteristics (tensile strength, elongation, etc.) according to the proportion of pearlite and ferrite in the matrix.

It is known that graphitization occurs at temperatures ranging between 800 and 1,000 C., the duration of this treatment being longer as the temperature is lower. Further, the ferritization treatment is effected either by cooling the objects between approximately 800 and 650 C. or by maintaining the objects for several hours at a temperature slightly less than the critical temperature, which is in the region of 750 C. The complete heat treatment of spheroidal graphite cast irons therefore takes much longer than the normal heat treatment for centrifugally cast pipes.

As is known, in the manufacture of pipes by centrifugal casting, where the objects are obtained in a continuous manner, the heat treatment is generally carried out in a continuous or tunnel furnace, the objects entering at one end and being fed towards the other end at a uniform rate. On account of the long duration of the heat treatment of spheroidal graphite cast irons, it is not possible to use ordinary cast iron pipe furnaces for pipes centrifugally cast in spheroidal cast iron. The length of the furnace is proportional to the duration of the heat treatment and as in the case of spheroidal cast iron the heat treatment has a duration which is much longer than that of centrifugally cast pipes in ordinary cast iron, the length of the furnace would need to be excessively long.

It might be proposed to effect the graphitization at a temperature in the region of 800 C. so that the objects have, at the end of this graphitization, a temperature as near as possible to the temperature of ferritization. But this would require a very long furnace owing to the very long duration of the graphitization (several hours). If, on the other hand, the duration of graphitization is reduced by carrying out this treatment at high temperature, the length of the furnace necessary for this first 2,872,175 Patented Feb. 3, 1959 ice treatment is reduced, but, on the other hand, a certain length of furnace is necessary for that part of the travel of the objects corresponding to their slow cooling before reaching the initial temperature of the ferritization treatment, and this length is the greater as the graphitization is siderably reducing the space necessary for the complete annealing treatment.

An object of the invention is to provide a furnace intended for annealing operations in two successive stages at temperatures differing from one another by a large temperature drop and in particular for the annealing of pipes and other tubular objects especially of spheroidal graphite cast iron cast in metal moulds by centrifugal casting or other methods, this furnace, which is of the tunnel type which the pipes or other objects, subjected to a continuous movement, enter at one end and issue from the other, comprising an input high temperature heating zone and an output zone for maintaining the temperature at a lower value, connected by a short intermediate zone which is highly cooled and is adapted to lower rapidly at the exit of the heating zone the temperature of the treated objects, whereby the latter are brought, at the entrance of the output zone, to the required pre-determined temperature which is notably less than said high temperature.

This intermediate zone for highly cooling the objects permits a considerable reduction in the distance between the input and output treatment zones without requiring a modified rate of feed of the objects in the furnace.

In the accompanying drawings given merely by way of example:

Fig. l is a longitudinal sectional view of a furnace embodying the invention, and

Fig. 2 is a sectional view taken along line 2-2 of Fig. 1, a portion of the furnace having been cut away.

In the illustrated embodiment, the tunnel furnace comprises a bottom or base 1, lateral walls 2, and a roof 3. Embedded in the base 1 are the runways 4 and disposed in grooves 5 are two endless chains 6. The latter have fingers 7 and are mounted at both ends on sprockets 8 connected to a common shaft, one of these shafts being driven by a reduction motor, not shoWn in the drawing. These chains are adapted to feed the objects to be treated, for example pipes. These pipes roll along the runways 4, for example in the direction of arrow 1 (Fig. 1). These objects enter through an entrance 10 at one end of the furnace and issue from the latter through an exit 11 at the other end. The openings 10 and 11 of the furnace may be advantageously closed by raisable doors 12 and 13.

Extending downwardly from the underside of the roof 3 are two transverse barrier walls 14 and 15 which form two sills whose positions will be determined hereunder. These barrier walls terminate a distance I from the base 1, so that they clear the objects 9 passing through the furnace.

These barrier walls 14 and 15 divide the interior of the furnace into three chambers or zones: an input heating chamber A, situated between the entrance 10 and the lower wall 14, an intermediate chamber B situated between the two lower walls, and an output chamber C situated between the lower wall 15 and the exit 11.

Communicating with the interior of the chamber A through the roof 3 and the side walls 2 is a series of burners 18 connected to a double pipe 19, 19 supplying combustible fluid, for example gas and air. In this pipe 1-9, 1-9 are mounted all'theusual conventional'devices for adjusting the flame and hence the temperature inside the chamber A.

Disposed in a transverse =plane'ofthe chamber B and in the roof 3 midway between the two barrier walls, is a series of passageways 20 connected to a pipe 21 supplying air or other cold gaseous fluid. Disposed in this pipe 21 is a'regulator or closing flap 22 which maybe actuated by a lever 23.

Communicating with the interior of the chamber C through the roof and lateral walls, is a series of burners 24 connected to a double pipe 25,25 supplying-gas and air. This double pipe 25, 25 is also supplied with'all the conventional devices for adjusting the flame and hence the temperature of'the chamber C (not shown in the drawing).

At both ends of the intermediate chamber B, i. e., substantially vertically below the barrier walls 14- and 15, are providedsuction pipes 26 and 27 which are connected respectively to collector pipes 28 and 29, in turn connected to a singlesuction fan 30 which discharges the fluid toa flue 30 Disposed in each of the collector .pipes 28 and 29 is a thermocouple 31 (Fig. 2) connected to an indicator 32 for measuring the output temperature T of the gases drawn into the chamber B. Further, a valve or flap 33, operated for example by hand by means of a lever 34, is

provided for each of these pipes.

Mounted in the ,roof of the chamber C in the vicinity of the barrier wall is a fully oiientable telescope or spyglass 35. This telescope is connected to a recording device 36 and permits recording, as a continuous curve of sinusoidal form, the temperature T of each object passing by the telescope on'the output side of the chamber B and the temperature vT of the base ofthe chamberlC, which is slightly different.

The furnace operates in a'very simple manner. The objects 9, introduced through the entrance 10 and travelling at constant speed, firstly enterthe chamber A, where they are brought during a given period of time corresponding to the rate of feed of these objects and the length of this chamber, to a desired graphitization temperature T this temperature being regulated by the conventional regulating means for the burners 18. The objects then enter the intermediate chamber B where they are rapidly cooled from temperature T a to temperature T by air or other cold gaseous fluid entering downwardly through 20. This gas, owing to the double suction obtained at 26 and 27, is dividedinto two divergent sheets n and n which ensures an excellent contact with the objects to be cooled. The objects at temperature T then enter the chamber C, their temperature being indicated by means of the telescope 35. Their temperature drops slightly to the temperature "l" of the chamber C, this temperature being slightlyllower than the temperature T The objects issue from the furnace at 11 at this temperature T The furnace is controlled, in so far as concerns the temperatures, in the following manner: Having determined the length of the chamber B, the number of objects 9 it is capable of containing and the rate of feed of the chain are known. Knowing the temperature T of the objects at the entrance and the temperature T at the'exit of this chamber, the number of calories to be removed by means of the cold fluid may be ascertained, and a heat calculation permits determining the required fluid how, the latter being regulated by the valve 22. Given the temperature T of the atmosphere, i. e., the input air, the temperature T of the heated air issuing from the passageways 26 and 27 may be calculated. Thus the suction valves 33 may be so set that the output temperature .T dridicated'by the .device.32 is equal to, or preferably slightlyhigher than, T

The temperatures T and T being known, it suflices -to verify that-the corresponding indications at 36 and 32 do not differ from these values and to bring them to the latter in the case of departure by acting on the valves 22 and 33 respectively.

The lengths of the chambersA and C are determined in accordance with the duration of each of the desired treatments, account ibeingtaken of'the rate of feed of the pipes through the furnace, i. e., the constant continuous displacement of the chain 6.

Thus in the furnace embodying the invention the graphitization treatment may be carried out at maximum temperature which permits, for a given quality of the cast iron to be treated, the shortest possible duration of treatment without it being necessary to provide an excessive length of furnace between the chamber A corresponding to the graphitization and the chamber C corresponding to the ferritization, notwithstanding the wide temperature difference between the two treatments.

Hence, if the graphitization is carried out at 950 C. and the ferritizatio-n commences at 800 C., the passage from 950to'800" 'C.,' narriclyd50 0, would take (allowing for a natural cooling in the furnace of about 1 C. per minute) about minutes in a conventional furnace, that is, a rate of displacement of the chain of, for exampic, 20 cm. per minute, and an inactive length of furnace of 30 meters. In the furnace embodying the invention the cooling period may be reduced to 5-10 minutes,that is for the same rate of displacement of the chain, a length of the chamber B of 1-2 meters. Hence there is an obvious saving in the cost of construction of the furnace.

It should be understood that the'foregoing values are only illustrative and that the lengths of the furnace vary with the number of objects to anneal, the rate of travel of the chain and the temperature difference to be obtained. However, it may be said'that if, owing to the blowing of the air-throughthe chamber B, it is possible to obtain cooling rates which are ten to twenty times faster than those obtained with natural cooling, it would be possible to reduce the length of the chamber B in the same proportion relative to the length it should have if only a natural cooling was relied upon, and this being, moreover, possible in limiting, by means of the barrier walls 14 and 15 and the pipes 26 and 27, the cooling to merely the length of the chamber B. Although a specific embodiment of the invention has been described hereinbefore, the invention is not limited thereto since many changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claim. Thus the cooling of the pipes or other objects passing through the chamber B may be supplemented by other devices, known per se, such as radiating panels which are cooled by circulated water and are disposed in the roof and walls of the chamber.

Having .now described my invention what I claim as new and desire to secure by Letters Patent is:

In an annealing tunnel furnace for pipes and like articles: two transverse barrier walls attached to the roof of said tunnel furnace and dividing said tunnel into three successive chambers consisting of a ferrititzing downstream chamber, an intermediate cooling chamber and a heated graphitizing upstream chamber; said intermediate chamher communicating with the other two chambers by free inlet and outlet passageways; the distance between said two barrier walls and the length of said intermediate chamber between said walls being not in excess of a few meters; in the roof of said tunnel, a series of orifices for admitting a cold fluid and disposed in a transverse plane passing through the middle of said intermediate chamber; in the bottom of said furnace and in said intermediate chamber, near said inlet and outlet passageways, two transverse rows of suction pipes; the cold 'fiuid entering said intermediate chamber through said series of roof orifices being divided after a short vertical course into two divergent sheets, which become longitudinal and horizontal to spread over the entire width of the furnace, each of said sheets escaping through one of said two rows of suction pipes whereby said articles passing through said intermediate chamber are cooled intensively from a graphitizing temperature of about 950 C. to a ferritization temperature not in excess of 800 C., in a time period of from about 5 minutes to about 10 minutes.

References Cited in the file of this patent UNITED STATES PATENTS Carpenter et a1. Apr. 20, 1929 Cope et al. Apr. 7, 1931 Baily June 2, 1931 Cope Aug. 9, 1938 Ette Aug. 30, 1938 Bonte Nov. 7, 1944 

